[DebugInfo][LoopStrengthReduction] SCEV-based salvaging for LSR

This patch extends salvaging of debuginfo in the Loop Strength Reduction
(LSR) pass by translating Scalar Evaluations (SCEV) into DIExpressions.
The method is as follows:
- Cache dbg.value intrinsics that are salvageable.
- Obtain a loop Induction Variable (IV) from ScalarExpressionExpander or
  the loop header.
- Translate the IV SCEV into an expression that recovers the current
  loop iteration count. Combine this with the dbg.value's location
  op SCEV to create a DIExpression that salvages the value.

Review by: jmorse

Differential Revision: https://reviews.llvm.org/D105207

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 5af1c37..3978e1e 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -1393,9 +1393,10 @@ SCEVExpander::getAddRecExprPHILiterally(const SCEVAddRecExpr *Normalized,
   // can ensure that IVIncrement dominates the current uses.
   PostIncLoops = SavedPostIncLoops;
 
-  // Remember this PHI, even in post-inc mode.
+  // Remember this PHI, even in post-inc mode. LSR SCEV-based salvaging is most
+  // effective when we are able to use an IV inserted here, so record it.
   InsertedValues.insert(PN);
-
+  InsertedIVs.push_back(PN);
   return PN;
 }